Militant groups, foreign states and criminal organizations pose a growing threat to U.S. security as they target government and private computer networks, FBI Director Robert Mueller said on Thursday.

In a speech to an Internet security conference, Mueller said militant groups like al Qaeda had primarily used the Internet to recruit members and plan attacks, but had made clear they also see it as a target.

"Terrorists have shown a clear interest in pursuing hacking skills and they will either train their own recruits or hire outsiders with an eye toward combining physical attacks with cyber attacks," Mueller said.

He noted a cyberattack could have the same impact as a "well-placed bomb."

Mueller added that some foreign governments, which he did not identify, also posed a threat by seeking to use the Internet for espionage.

"Apart from the terrorist threat, nation-states may use the Internet as a means of attack for political ends," he said.

"Nation-state hackers or mercenaries for hire" as well as rogue hackers or international criminal syndicates are targeting government networks, Mueller added.

The comments came in the wake of several international Internet security incidents.

In January, Google Inc, the world's No. 1 Internet search engine, said it had detected a sophisticated online attack on its systems that originated in China and said it believed at least 20 other companies had been targeted.

According to Google, one of the primary goals of the attacks was accessing the personal e-mail accounts of Chinese human rights activists.

Earlier this week, Spanish police arrested three men accused of masterminding one of the largest computer crimes to date, in which more than 13 million PCs were infected with a virus that stole credit card numbers and data.

Mueller said international cooperation was essential to combating online crime like the so-called Mariposa botnet incident in Spain. He added the FBI had 60 "attache" offices around the world as well as special agents embedded with police forces in countries such as Romania, Estonia and the Netherlands.

He urged businesses targeted in cyberattacks to come forward to help track down the perpetrators, saying the FBI was attuned to the delicate nature of the situation for corporations.

"We will minimize the disruption to your business, we will safeguard your privacy and your data and where necessary we will seek protective orders to preserve trade secrets and business confidentiality," he said.